Staphylococcus aureus (S. aureus) is one of the most frequent causes of bacterial infections in the U.S. and is responsible for diverse types of infections ranging in severity from mild to fatal in both hospital and community settings. In the community, S. aureus typically causes skin-and-soft-tissue infections with an estimated 12 ? 13 million outpatient visits per-year (in the U.S.) but can also cause severe manifestations including necrotizing fasciitis and sepsis in individuals with no underlying risk. One of the most clinically significant aspects of S. aureus infections is the high prevalence of drug resistance and the innate ability for S. aureus to develop resistance to antibiotics making it very difficult to develop therapeutics that will have potential for long-term efficacy on S. aureus. The research proposed in this application will broaden our understanding of host- pathogen interactions, providing the foundation for future studies aimed at the intelligent design of novel vaccines and therapies to treat bacterial infection. Research that defines protective host immune responses and the mechanisms used by pathogens to undermine them can be translated to new treatment strategies that will improve public health. Based on strong published and preliminary findings using mouse models of infection and human neutrophil assays, we have identified specific host and pathogen factors that will be investigated to determine their precise roles in pathogenesis. To that end, this project will study how S. aureus uses the SaeR/S two-component gene regulatory system to evade innate immunity. The SaeR/S system is essential for evasion of neutrophil killing; however, exact mechanisms dependent on SaeR/S resulting in neutrophil dysfunction are not defined. Three specific aims will test the hypothesis that S. aureus uses the SaeS/R sensory system to sense and adapt to neutrophil challenge. Experiments outlined in Specific Aim 1 will use host-pathogen NGS RNA-seq to identify human neutrophil components influenced by SaeR/S, novel SaeR/S regulated virulence factors induced upon neutrophil exposure, as well as resolve the influence of individual components of the sae system (saeP, saeQ, saeR, and saeS) on virulence regulation. Preliminary data demonstrates our ability to successfully perform these technically challenging experiments. Specific Aim 2 of this proposal will test an innovative hypothesis that a newly identified SaeR/S-dependent virulence factor inhibits neutrophil reactive oxygen species, independent of previously defined mechanisms, via targeting human myeloperoxidase. Experiments outlined in Specific Aim 3 will explore unique findings that indicate SaeR/S-regulated factors manipulate IFN? and IL-17A production during skin and soft-tissue infection, driving host immunity towards a TH1-type immune response associated with increased pathogenesis and away from a protective Th17-type response. Collectively, this study will advance understanding of sense-and-response adaptations by S. aureus to improve our knowledge of the initial host-pathogen interactions that lead to S. aureus infection. Results will provide valuable information for vaccine and immunotherapeutic development.